1. Field of the Invention
The invention relates in general to sheet positioning devices and in particular to a new and useful arrangement for the edge or center control of sheets or webs comprising a motor pump unit for generating an air supply and the fluid pressure for a diaphragm drive, and an adjusting device for a pneumatic web edge sensor.
2. Description of the Prior Art
In such devices, the web edge sensor is supplied with air from the motor pump unit, where the position of the running web edge is determined and an air pressure proportional to the position of the web edge is applied as a receiver pressure on the diaphragm drive. This, in turn, conducts a fluid current proportional to the receiver pressure to the adjusting device and displaces the web edge in such a way that it returns into its nominal position. Web edge controls of the above described type are known from U.S. Pat. No. 3,244,340, for example, as well as from German Pat. No. 1,574,638. But the web edge control device according to German Pat. No. 1,574,638 has the disadvantage that the zero point of the diaphragm drive can be changed by atmospheric air pressure fluctuations or by speed fluctuations of the compressor and/or by contamination of the filters, so that the zero point of the adjusting device for the web is also displaced. Besides, it is not possible in this control device to adjust the steepness. If the steepness of the control circuit is varied in the known control device according to U.S. Pat. No. 3,244,340 by varying the supply air pressure, the zero point of the diaphragm drive also changes and thus also the zero point of the adjusting device. Readjustment of the zero point is thus necessary in both known devices, which is effected by shifting a spring on the diaphragm drive. If the supply air is cut off, this results in a considerable zero point error. Besides, it is not possible to change from web edge control to web center control without a major design effort.